1. Field of the Invention
The present invention relates to a method for efficiently incorporating light absorbing compounds into a polyester composition. More particularly, the present method relates to incorporating a nitrogen containing methine light absorbing compound into a polyester composition.
2. Background Art
Polyester is a polymeric resin widely used in a number of packaging and fiber based applications. Poly(ethylene terephthalate) (“PET”) or a modified PET is the polymer of choice for making beverage and food containers such as plastic bottles and jars used for carbonated beverages, water, juices, foods, detergents, cosmetics, and other products.
In the typical polyester forming polycondensation reaction, a diol such as ethylene glycol is reacted with a dicarboxylic acid or a dicarboxylic acid ester. The reaction is accelerated by the addition of a suitable reaction catalyst. Since the product of the polyester condensation reaction tends to be reversible and in order to increase the molecular weight of the polyesters, this reaction is often carried out in a multi-chamber polycondensation reaction system having several reaction chambers operating in series. Typically, the diol and the dicarboxylic acid component are introduced in the first reactor at a relatively high pressure. After polymerizing at an elevated temperature the resulting polymer is then transferred to the second reaction chamber which is operated at a lower pressure than the first chamber. The polymer continues to grow in this second chamber with volatile compounds being removed. This process is repeated successively for each reactor, each of which are operated at lower and lower pressures. The result of this step wise condensation is the formation of polyester with higher molecular weight and higher inherent viscosity.
During the polycondensation process, various additives such as colorants and light absorbers may be added. Light absorbers are an important additive, both for imparting stability to the polyesters and to protect products packaged in PET containers from degradation induced by exposure to visible and UV light. U.S. Pat. No. 4,617,374 (hereinafter '374 patent) discloses the use of certain UV-absorbing methine compounds that may be incorporated in a polyester or a polycarbonate during polycondensation. These compounds enhance ultraviolet or visible light absorption with a maximum absorbance within the range of from about 320 nm to about 380 nm. Functionally, these compounds contain an acid or ester group which condenses onto the polymer chain as a terminator. Moreover, the UV absorbers of the '374 patent have been found to be useful in the preparation of polyesters such as poly(ethylene terephthalate) and copolymers of poly(ethylene terephthalate) and poly(1,4-cyclohexylenedimethylene terephthalate). It has been observed, however, that some light absorbing compounds are somewhat volatile causing the yield of these light absorbers in the formed polyester to be somewhat less than 100% (values of 80% to 85% are typical). Moreover, these compounds may plug the equipment by condensing in the process lines. The loss of light absorber results in added costs for the polyester formation because of the down time needed to clean process lines and because of the relatively high cost of these compounds.
Accordingly, there is a need for improved methods of incorporating light absorbing compounds into polyester compositions made by the melt phase polycondensation method, and/or improved polyester compositions containing light absorbers.